Iron and Molybdenum Team Up to Shield Cobalt Catalyst From Corrosive Seawater

Hydrogen has long been heralded as the clean fuel of the future, but the way most of it is made today is anything but clean. More than 95 percent of the world’s hydrogen supply still comes from steam reforming of methane and natural gas, a process that releases large quantities of carbon dioxide and directly undermines global efforts toward carbon neutrality. Electrochemical water splitting offers a genuinely green alternative: pass electricity from renewable sources through water, and the hydrogen evolution reaction at the cathode and the oxygen evolution reaction at the anode deliver pure hydrogen and oxygen with no carbon footprint. The catch is that this elegant chemistry demands catalysts that are simultaneously cheap, active, and tough enough to survive years of continuous operation. Platinum, iridium, and ruthenium catalysts perform superbly in the laboratory, yet their scarcity and cost make them poor candidates for the gigawatt-scale electrolyzers the energy transition will require.

A research team led by Jinhyuk Baek, Yujin Son, Moonsu Kim, and Gibaek Lee at Yeungnam University and Gyeongkuk National University in the Republic of Korea has now reported a promising way out of this dilemma. Writing in the journal Advances in Industrial and Engineering Chemistry, they describe a ternary iron-molybdenum-cobalt oxide electrocatalyst, abbreviated Fe-CMO, that not only matches the activity of noble-metal benchmarks in alkaline water but also shrugs off the chloride ions that make seawater so hostile to conventional catalysts. Because seawater accounts for more than 97 percent of the water on Earth, a catalyst that can split it directly would remove one of the biggest resource constraints on green hydrogen production, freeing coastal regions and island nations from dependence on scarce freshwater supplies.

The problem with seawater is deceptively simple. Its roughly 0.5 molar concentration of chloride ions creates a competing reaction at the anode: instead of oxidizing water to oxygen, the electrode can oxidize chloride to chlorine or hypochlorous acid. This chlorine evolution reaction steals current, lowers efficiency, and generates corrosive byproducts that chew through catalyst surfaces. Most high-performing catalysts developed for alkaline freshwater electrolysis degrade rapidly when immersed in saline media, which is precisely why so many seawater-splitting studies report impressive initial numbers followed by disappointing stability. The Korean team’s strategy was to attack the chloride problem at the level of electronic structure, using two dopants with complementary roles rather than a single additive.

Synthesis began with a two-step hydrothermal process on a nickel foam substrate. In the first step, cobalt nitrate, urea, and ammonium fluoride were combined to grow urchin-like arrays of cobalt layered double hydroxide nanoneedles, with the urea and fluoride acting as structure-directing agents that produce a porous, radially aligned framework. The second hydrothermal step introduced sodium molybdate and iron nitrate, after which the sample was annealed first at 280 degrees Celsius and then at 450 degrees Celsius under a reducing 90:10 argon-hydrogen atmosphere. The sequence matters enormously. When the cobalt hydroxide precursor was annealed directly, or when iron alone was added, calcination caused the nanoneedles to coalesce into dense coral-reef-like structures, collapsing the surface area that electrocatalysis depends on. Molybdate ions, by contrast, transformed the nanoneedles into interconnected nanosheets, and the co-doped final product emerged as a well-defined three-dimensional nanoflower built from entangled nanosheet clusters that resist structural collapse during thermal treatment.

Microscopy and spectroscopy confirmed that the nanoflower is a genuine spinel cobalt oxide with the dopants incorporated at the lattice level rather than merely sitting on the surface. High-resolution transmission electron microscopy revealed lattice fringes of 0.25 and 0.29 nanometers corresponding to the (311) and (220) planes of spinel Co3O4, and elemental mapping showed a homogeneous distribution of cobalt, molybdenum, and iron throughout the material. X-ray photoelectron spectroscopy then delivered the most chemically interesting result: the coexistence of multiple oxidation states, including Mo4+, Mo5+, and Mo6+, alongside Co2+/Co3+ and Fe2+/Fe3+ redox couples. Critically, when the team annealed a comparison sample under argon alone, the Mo4+ signal vanished entirely, proving that the reducing atmosphere is essential for stabilizing the lower molybdenum valence that drives hydrogen evolution.

The division of labor among the three metals is what gives Fe-CMO its bifunctional character. Molybdenum6+ acts as an electron acceptor, pulling electrons from cobalt sites and continuously regenerating the Co2+/Co3+ redox pair that powers the oxygen evolution reaction. Molybdenum4+ plays the opposite role on the cathode side, serving as an electron reservoir that promotes the conversion of adsorbed hydrogen intermediates into molecular hydrogen at Mo-edge sites. Iron, present at a modest 1.31 atomic percent, modulates the local electronic environment around cobalt centers: the cobalt peaks in the XPS spectrum shift toward higher binding energies relative to the undoped material, indicating elevated positive charge on cobalt that favors the adsorption of anionic intermediates such as hydroxide. Raman spectroscopy corroborated this picture, with the A1g vibrational mode near 650 wavenumbers shifting leftward in the doped samples, a signature that iron substitution increases the covalency of the cobalt-oxygen bonds. The Fe2+/Fe3+ couple also provides local charge balance that keeps Mo4+ stable under operating conditions, preventing the electron reservoir from being irreversibly oxidized.

The electrochemical numbers are striking. In 1 molar potassium hydroxide, Fe-CMO required overpotentials of just 231 millivolts at 10 milliamperes per square centimeter and 289 millivolts at 50 milliamperes per square centimeter for oxygen evolution, comfortably beating commercial IrO2 and all undoped or singly doped controls. Its Tafel slope of 190 millivolts per decade was the lowest measured, and electrochemical impedance spectroscopy showed a charge transfer resistance of only 0.39 ohms, roughly one-eighth that of plain Co3O4. On the hydrogen side, the catalyst needed 153 millivolts at 50 milliamperes per square centimeter, nearly matching the commercial Pt/C benchmark at 129 millivolts, with a Tafel slope of 79 millivolts per decade characteristic of a Volmer-Heyrovsky pathway in which electrochemical desorption is rate-determining. The double-layer capacitance measurements told a structural story as well: Fe-CMO achieved 53 millifarads per square centimeter, compared with just 3 for the collapsed coral-like Co3O4, confirming that the molybdenum-guided nanosheet architecture preserves a far larger pool of accessible active sites.

When the electrolyte was switched to a mixture of 1 molar potassium hydroxide and real seawater collected from the East Sea at Najeong Beach in Gyeongju, the catalyst barely flinched. Oxygen evolution overpotentials of 238 millivolts at 10 and 289 millivolts at 50 milliamperes per square centimeter were essentially identical to the freshwater values, while hydrogen evolution required only 98 millivolts at 50 milliamperes per square centimeter, actually edging out Pt/C under the same conditions. In a two-electrode configuration pairing Fe-CMO against itself, the cell maintained nearly identical current-voltage characteristics in alkaline and seawater-mixed solutions, whereas undoped Co3O4 suffered severe degradation in the presence of chloride. Control experiments isolated the culprit and the cure: iron doping alone conferred little protection against chloride corrosion, but molybdenum incorporation dramatically improved stability, pointing to molybdenum’s ion-selective behavior and its ability to suppress chloride adsorption as the decisive factor.

Durability testing sealed the case. After 120 hours of continuous operation at an industrially relevant current density of 100 milliamperes per square centimeter in seawater-containing electrolyte, the cell voltage rose by only 19 millivolts, a performance loss of just 1.1 percent, while undoped Co3O4 lost 115 millivolts under far milder conditions. Post-mortem X-ray photoelectron spectroscopy of both electrodes revealed a fascinating asymmetric chemistry: on the anode side, molybdenum had partially oxidized from Mo4+ to Mo5+, cobalt’s metallic signal had disappeared into higher-valent states, and iron remained locked in the Fe3+ state, all consistent with productive oxygen evolution rather than parasitic chlorine chemistry. On the cathode side, reduced molybdenum and a healthy Co2+/Co3+ ratio were preserved, showing that each half of the cell maintains the valence environment its reaction requires. This reversible, reaction-specific redox adaptability is precisely what long-term seawater electrolysis demands, and it demonstrates that rational multimetal design, rather than exotic materials or prohibitively expensive noble metals, may be the key to unlocking the oceans as a hydrogen feedstock.

Subject of Research: Development of a chloride-resistant Fe-Mo co-doped Co3O4 electrocatalyst for hydrogen production via seawater electrolysis

Article Title: Robust Fe, Mo co-doped Co3O4 electrocatalyst with superior chloride resistance for efficient seawater electrolysis

Article References: Baek, J., Son, Y., Kim, M., & Lee, G. (2025). Robust Fe, Mo co-doped Co3O4 electrocatalyst with superior chloride resistance for efficient seawater electrolysis. Advances in Industrial and Engineering Chemistry, 1(1), Article 18. https://doi.org/10.1007/s44405-025-00018-9

Image Credits: AI Generated

DOI: 10.1007/s44405-025-00018-9

Keywords: seawater electrolysis, green hydrogen, electrocatalyst, cobalt oxide, molybdenum doping, iron doping, oxygen evolution reaction, hydrogen evolution reaction, chloride resistance, spinel structure, bifunctional catalyst, water splitting